1. Field of the Invention
The present invention relates to a method for inspecting a photomask. More particularly, the present invention relates to a real-time online method for inspecting a photomask.
2. Description of Related Art
Recently, semiconductors have been developed towards the design trend of miniaturizing the circuit components, and one of the most important steps in the whole semiconductor process is photolithography process. As for critical dimension (CD) relevant to the structure of the semiconductor device, the pattern of each thin film, for example, is determined by the photolithography process. Therefore, the accuracy of pattern transfer from the photomask to the wafer plays a crucial role. If the pattern on the photomask is not accurate, the pattern transfer is made to be further away from being accurate, which thus affects the tolerance of the CD on the wafer, and reduces the resolution of the exposure.
In a general fabrication of the photomask, sulfuric acid is usually used to clean the photomask, such that residual sulfate ions (SO42−) remain on the photomask. Then, during the photolithography process, an exposure is performed by the photomask to transfer the pattern on the photomask to the wafer, and the sulfate ions (SO42−) on the photomask and ammonium ions (NH4+) are easily bonded with each other, or other particles may be adhered on the photomask, so as to result in a haze on the photomask. The haze formed at the beginning is quite small in size, which has barely effect on the process. However, as the process time prolongs or as the times for performing the photolithography process increases, the chemical change continuously occurs on the photomask, which causes the haze to be gradually enlarged to further affect the pattern on the photomask. If the photomask with the enlarged haze is used to perform the pattern transfer, the pattern transferred to the wafer is usually inaccurate, which seriously affects the subsequent process.
Generally, in order to determine whether the photomask has a haze or not, a photomask inspecting tool is used to scan the photomask. However, before using the photomask inspecting tool to inspect the photomask, it is necessary to take out the photomask from the equipment to perform an off-line photomask inspection. In this manner, it has to consume time in waiting for the result of the inspection, and the sampling rate of the inspection is relatively low. Furthermore, the photomask inspecting tool is usually very expensive, such that the cost is greatly increased.